The invention concerns an internal combustion engine, in particular a stationary gas engine, comprising a compressor arranged in the induction tract, an electrically operable blow-off valve connected downstream of the compressor, and a pressure sensor for detecting the pressure actual value of the mixture in the region of the blow-off valve.
In order to improve the load shut-down characteristics in the case of engines, in particular gas engines, it is already known for a blow-off valve to be connected downstream of the compressor in order to be able to bring down the charging pressure in load removal situations without turbocharger pumping. Turbocharger pumping occurs whenever the compressor is xe2x80x98throttledxe2x80x99 at a working point with a great deal of throughput on the outlet side. That procedurexe2x80x94once startedxe2x80x94is totally uncontrollable and generally results in xe2x80x98total lossxe2x80x99 of the charging pressure with subsequent stoppage of the engine. In order to prevent an excessive increase in the engine speed in the event of load shut-down however, xe2x80x98throttlingxe2x80x99 must be effected. Therefore, by means of blowoff regulation, a valve downstream of the turbocharger is opened virtually at the same moment in time and that therefore affords the possibility of being able to reduce the throughput in a controlled fashion (in spite of closure of the throttle valve). U.S. Pat. No. 5,526,645 discloses an excess air control system for a dual-fuel or spark ignition gas internal combustion engine in which an air blow-off valve is coupled to an air bypass-line linking the air intake to a turbo charger and the air manifold in order to control the air manifold pressure dependence on different measured engine characterisation parameters. This concept cannot be used for improving the load shut down characteristics of an internal combustion engine of the kind set forth which is mainly controlled by a throttle valve, because blowing an air-gas-mixture around a turbo charger in a bypassline results in a negative impact on the air-gas-mixture and is therefore not practicable.
The object of the invention is to provide an internal combustion engine of the kind set forth in the opening part of this specification for reliable regulation for the blow-off valve, in particular for the avoidance of turbocharger pumping.
The internal combustion engine according to the invention is characterised by:
a device for detecting the engine power output,
a device for calculating a pressure reference value in dependence on the detected engine power output and a relationship previously stored in a memory between engine power output and pressure reference value, and
a regulating device for regulating the blow-off valve until the pressure actual value reaches the calculated pressure reference value.
The invention is based on the notion that, at a given combustion gas-air ratio of the mixture (and at a given temperature) a predetermined relationship obtains between the engine power output and the pressure of the mixture in the induction tract. In accordance with the invention, that relationship is utilised to afford a pressure-controlled regulation effect for the blow-off valve The relationship between engine power output and the pressure reference value can be ascertained either empirically or from measurements. It can then be stored for example in the form of a formula with given parameters or in the form of a cross-reference table. At any event that arrangement is capable of achieving a pressure reference value from the actually measured engine power output and a correctively involved temperature value.
The blow-off valve is now regulated until the pressure actual value reaches the pressure reference value. In general, that pressure reference value above which the blow-off valve opens is set higher by a small amount (for example between 0.2 and 0.3 bar) than corresponds to the empirical and measured relationship between the values involved, in the case of an engine without blow-off valve. That therefore advantageously provides with the blow-off valve a safety margin in relation to the set pressure point, so that a normal engine regulation system which acts on the throttle valve still has an adequate effective range of influence. When the throttle valve is completely open therefore the engine speed still cannot be brought under control although a large part of the xe2x80x98over-powerxe2x80x99 is removed by way of the blow-off valve The actual reduction in excessive speed of rotation then has to be effected by way of the above-mentioned throttle valve control system. It will be appreciated that the throttle valve does not now have to be closed very far because of the pressure which has been kept at a low level, in order to be able to reduce the engine power output under the given load.
If the pressure downstream of the compressor which is in particular an exhaust gas turbocharger falls below the reference value the blow-off valve is closed, which means that as much mass flow as possible again passes to the engine and also to the turbine. As a result, the engine power output and also the turbocharger speed can be held at a sufficiently high level so that the engine speed does not fall excessively.
For shutdown procedures in the range of low levels of power output, a minimum reference value is predetermined for the pressure so that the valve does not remain constantly open in an overspeed situation and when the pressure is already low. By virtue of the regulating procedure, no compressor pumping occurs with a sufficiently large blow-off valve cross-section and sufficiently rapid opening of the valve, as the blow-off valve holds the charging pressure at a low level in the ranges in which the throttle valve can close far by virtue of the overspeed and in which therefore there is a pumping risk.
Preferably therefore for that purpose there is further provided a device for detecting the speed of the engine, wherein a comparison device closes the blow-off valve independently of the signal from the regulating device when the speed is or falls below a predeterminable reference value. The regulating system according to the invention remains a pressure-controlled regulating system. The speed signal is used independently of that pressure-controlled regulation effect to close the blow-off valve when the speed falls below a reference value.
Further advantages and details of the invention will be described in greater detail with reference to the specific description hereinafter.